Elsevier

CATENA

Volume 107, August 2013, Pages 145-153
CATENA

Flow hydraulic characteristic effect on sediment and solute transport on slope erosion

https://doi.org/10.1016/j.catena.2013.03.001Get rights and content

Highlights

  • Data in experiments for two series of soil surface conditions were compared.

  • Flow hydraulics, sediment, and solute transport in soil erosion were concerned.

  • Rate of Manning roughness coefficient to mean flow depth (n/h) is a good hydraulic indicator for sediment and solute.

  • Parameter of n/h represents the energy for water and sediment in erosion.

Abstract

Different hydraulic parameters, including the hydraulic shear stress, unit length shear force, steam power, unit steam power, and effective stream power were used to quantify flow detachment. Most former studies were conducted for flow detachment under uniform slope surface conditions, while a few studies compared different slope surface conditions. The uniform bare loess was prepared in laboratory experiments. Natural fallowed soil loess with stone covers was prepared in field experiments. The objective of this study was to assess the differences in hydraulic parameters and sediment detachment under these different soil surface conditions. Our results show that the unit sediment load (Rs) has a good linear relationship with the unit runoff rate (Rr) for the flume and field experiments, and the relationship can be expressed as the function: Rs = 0.262Rr  0.802 (R2 = 0.947). The rate of Manning roughness coefficient to mean flow depth (n/h) is a good hydraulic indicator like as the stream power and Reynolds number for predicting the sediment load. Hydraulic parameters n/h, Re, and ω are good indicators for the unit area sediment load for both the flume and field experiments, while Fr, f, and τ are good indicators for the unit area sediment load only when the flume experiments and field experiments are individually analyzed. Among the three good indicators (ω, Re, and n/h), n/h is better than the other two for predicting sediment load in rill erosion for both flume and field experiments, as well as for the unit solute transport rate (MBr). The parameter of n/h probably is not only a good hydraulic parameter as an indicator for both sediment and solute transport, but also a good hydraulic parameter which link with runoff energies. The parameter n/h represents the flow wave of runoff and is an important factor to represent the energy for water and sediment transport, and the flow wave celerity (vw) is related to n/h by: vw = 1.585(n/h) 0.527 (R2 = 0.978).

Introduction

Soil surface conditions such as surface roughness, vegetation and surface cover play important roles in rill erosion (Abrahams et al., 1998, Cerda, 1999, Epstein et al., 1966, Foster, 1982, Gimenez and Govers, 2002, Helming et al., 1998, Johnson et al., 1979, Nearing et al., 1990, Poesen et al., 1990, Romkens et al., 2001). Different soil surface conditions have different flow hydraulic resistances, even with the same initial conditions. Materials such as vegetation (Prosser and Dietrich, 1995) and stones (Abrahams et al., 2000) on the soil surface induce additional resistance to detachment by runoff.

There are two sorts of resistances, flow resistance and resistance to erosion, both of them play important roles in soil erosion process studies. Resistance to erosion is important for sediment transport, and which is usually quantified by e.g. cohesion or aggregate stability. Resistance to flow is quantified using e.g. Manning's n, Darcy–Weisbach equation and the Manning equation:f=8gRSV2n=V1S1/2R2/3where: g is acceleration of gravity (m·s 2), R is hydraulics radius (m), S is average slope (sine of slope angle), and V is the average flow velocity (m·s 1).

From the two equations, Darcy–Weisbach friction factor (f) is the most often used parameter to represent hydraulic resistance in overland flow studies conducted in the laboratory and field; while the Manning roughness coefficient (n) is the most widely used parameter in channel flow for engineering and construction purposes. In fact, both methods are based on the same important variables: the flow velocity and the water depth. Takken and Govers (2000) noted that the Manning roughness coefficient is likely to behave in the same way as Darcy–Weisbach friction factor. Hessel et al. (2003) observed similar results from their studies on erosion in steep loess slopes.

Most studies show that Darcy–Weisbach friction factor (f) is closely related to the corresponding Reynolds number (Re) for overland flow, but the relationship is different in different studies. Savat (1980) used sand and loess soils to study hydraulic resistance from grain friction in a laboratory flume. His result shows that the hydraulic grain roughness decreases as the Reynolds number (Re) increases. Rauws and Govers (1988) showed that for a non-erodible bed with artificial roughness elements glued to a flat bed, the relationship between Re and f varies as a function of slope. Gilley et al. (1990) measured the flow velocities in rills for 10 soils under a constant slope in a field study, and their results show that f is the negative exponential function of Re for each soil. Gilley et al. (1992) and Prosser et al. (1995) showed that f increases as Re increases under the condition of flow depth less than the size of the physical roughness elements. Nearing and Norton (1997) used sand and loess to study f change with slope using constant hydraulic radius, and they found that f decreases as Re increases. Hessel et al. (2003) showed that both f and n can be estimated from the Reynolds number.

The flow detachment in an erosion process is often described by energy-based approaches. Lye and Smerdon (1965) are among the earliest who used a hydraulic flume to investigate the relationship between soil erosion and hydraulic shear stress. They used the following equation to represent the soil detachment:τ=pgRSwhere: ρ is water mass density (kg·m 3), and τ is the hydraulic shear stress.

Later studies by Hairsine and Rose (1992a) showed that the shear stress of runoff was not a good predictor for soil detachment, and the stream power of flow (or the unit stream power) was a better one. The stream power (ω) is the energy of the flow dissipated to the flow boundary and was calculated by (Bagnold, 1977):ω=τV=pgRSVwhere: ω is stream power (kg·s 3), and V is the average flow velocity (m·s 1).

The above two hydraulic parameters, flow hydraulic shear stress (τ) and stream power (ω) both represent the flow energy dissipation to the boundaries (Nearing and Norton, 1997, Nearing et al., 1991, Hairsine and Rose, 1992a, Hairsine and Rose, 1992b). Nearing et al. (1991) studied soil detachment by shallow flow at low slopes with two soils and indicated that detachment rate for a given soil material was not a function of either shear stress or stream power of the flow. Leonard and Richard (2004) indicated that total shear stress of runoff is a poor predictor of detachment on rough soils. Elliot and Laflen (1993) found that stream power was the best hydraulic parameter for predicting detachment capacity of rill. Nearing and Parker (1994) showed that soil detachment by shallow flow at a given shear stress depends on flow turbulence, and detachment by turbulent flow is much greater than for laminar flow. Nearing and Norton (1997) used six series of experiments to study hydraulics and erosion in eroding rills and found that the Reynolds number is not a consistent predictor of hydraulic friction, while stream power is a consistent and appropriate predictor for unit sediment load for the entire data set. Their detachment rates are best correlated to a power function of either shear stress or stream power. Zhu et al. (1995) and Zhang et al. (2003) studied the mechanism of soil detachment by shallow flow and found that the linear relationship between detachment rate and shear stress gives a poorer prediction than a power function does, and stream power is a better parameter to predict detachment than shear stress. Gimenez and Govers (2002) studied the flow detachment on rough and smooth beds with two types of experiments and showed that unit length shear force and shear stress are the most universal detachment predictors.

These contrasting results in selecting hydraulic functions and their parameters to predict flow detachment occur perhaps because of different conditions in the different experiments, which implies that we still do not fully understand the fundamental mechanism of surface erosion. Moreover, most of the previous studies focused only on the relationship between sediment and flow hydraulics, and few evaluated the relationship between solute transport in runoff and flow hydraulics. Soil erosion in slope may be decided by two important aspects. One aspect is characters of soil itself, including soil type, soil surface vegetation, soil surface covers, etc. All these characters contribute to soil surface roughness. The representative factor we chose is Manning roughness coefficient (n). The other aspect is characters of flow, including flow velocity and flow depth. We choose the flow depth. At the same time, the former researches mainly focus on matter transport properties on slope in lab or in field, and there is a short comprehensive hydraulic parameter to describe water, sediment and solute on slope in lab and field. The main work of this article is to understand if the compound factor of n/h is a suitable hydraulic character to predict sediment and transported solute for different slope conditions. The objectives of this study were: (i) to compare flow hydraulics, sediment and solute transport in soil erosion under disturbed uniform loess slope in laboratory and natural fallowed loess slope land in field plot; (ii) to compare the mechanism of soil detachment and solute transport under the two soil surface conditions; (iii) to identify a suitable hydraulic parameter that can be widely used to predict sediment and surface solute transport in erosion under different soil surface conditions.

Section snippets

Materials and methods

Two series of soil surface conditions were prepared in experiments: (A) disturbed and uniform bare loess slope in the laboratory; (B) natural fallowed loess slope land in the field (natural fallowed loess slope land with or without stone covers). As described below, the laboratory flume experiments used the inflow water scouring + simulated rainfall, and the field experiments used inflow water scouring only. The particle size distribution of the test soil for flume and field experiments were

Flow hydraulics in flume and field experiments

The surface flow velocities of both flume and field experiments were measured during each run and the average flow velocities and other hydraulic parameters such as average flow depth, Reynolds number (Re), Froude number (Fr), Manning roughness coefficient (n), and Darcy–Weisbach friction factor (f) were calculated by Eqs. (1), (2), (5), (6), and (7) respectively and are summarized in Table 3.

The average flow velocities range from 0.30 m·s 1 to 0.44 m·s 1 in flume experiments, and from 0.16 m·s 1

Conclusions

Our experiments show that the rate of Manning roughness coefficient to mean flow depth (n/h), the Reynolds number (Re), and the stream power (ω) are good hydraulic indicators for the unit sediment load (Rs) for both the flume and the field experiments, while the Froude number (Fr), Darcy–Weisbach friction factor (f), and hydraulic shear stress (τ) can serve as indicators for Rs individually for flume experiments or for field experiments. The linear relationships between Rs and ω, and between Rs

Acknowledgments

This work was partially supported by research projects under the Key Project of National Natural Science Foundation of China (project No. 51239009), the National Natural Science Foundation of China (project Nos. 41101257 and 41171221), the Guangdong Natural Science Foundation of Partnership Program for Creative Research Teams (project No. S2012030006144), and the 973 Program (project No. 2011CB411903). We thank for the assistance of the Institute of Soil and Water Conservation of Chinese

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